Electrical apparatus in an underground case

ABSTRACT

Fluid filled electrical apparatus, such as transformers, suitable for underground or vault mounting. The electrical apparatus includes a laminated tank or casing, having at least three metallic layers, with the layer materials and thicknesses being selected to provide a good structural material having long service life in corrosion conducive environments, without undue economic penalty.

United States Patent Inventor Harry R. Sheppard Sharon, Pa. App]. No.43,443 Filed June 4, 1970 Patented Aug. 31, 1971 Assignee WestinghouseElectric Corporation Pittsburgh, Pa.

ELECTRICAL APPARATUS IN AN UNDERGROUND CASE 9 Claims, 5 Drawing Figs.

0.8. CI 174/17 R, 204/196, 336/94 Int. Cl 1101f 27/02 Fieldof Search174/17 R,

References Cited UNlTED STATES PATENTS 3,390,225 6/1968 Couch 174/373,405,283 10/1968 Leonard 174/37 X Primary Examiner-E. A; Goldberg vAttorneys-A. T. Stratton, F E. Browder and Donald R.-

Lackey PATENTED m1 l97l FlG.3.

I FiGfS.

INVENTOR I Harry R. Sheppard j Z/Jfll/ ATTORNEY ELECTRICAL APPARATUS 1NAN'uNoERcRounn'cAsE" BACKGROUND OF THE-INVENTION specially constructedconcrete vaults, seriously limit theuseful service life of such'apparatus, compared'withthe life of similar apparatus mounted'abovegrade. Power transformers of the network type are usually mounted inunderground vaults, with the conventional approach to corrosionprotectionbeing'to coatthe external tank surfaceswithspeciallyformulated coal tar epoxyresin systems. Protectivecoatings,however, areonly' as good-as theuniformity of the coatin g wwithanypin'holes or scratches in the coating, such as-thosedueto handlingand installing the apparatus, causing more rapid corrosion at the.exposed areas than if the tank had no protective coating. The use-of acorrosion resistant material. for theitank, such as stainless steel ornodular iron, while extending the-service life of the apparatus comparedwith mild steel .tanks, has'th'e.

disadvantageof a substantially higher cost. Even with corro-- sionresisting materials, separate cathodic protection, such as a sacrificialanode constructed of zinc and connected to the tank, is required toprotect the tank material from corrosion,

tanks or casings for both the vault 'mounted networlt'.trans-'- former,and the underground residentialmtype of distribution transformer,whether vault mounted or directly -buried,"as well as for any other typeof fluid filled electrical apparatus which is to be mounted in a highlycorrosive environment:

SUMMARY'OFTHE-INVENTION Briefly, the present invention is anewandimproved fluid filled electrical'apparatus of the type suitable 'forvault or.

below grade mounting. The electrical apparatus includes a. tankformed ofa laminated material, havingatleastthree metallic layers. The layeradjacent'the fluid in the tank, .i.e., the innermost or first layer of:the laminated'structure, is

selected forits structural properties and cost, without regard.

to its ability to resist corrosion, and thus this layer may be a.

milder plain carbon steel. The next or second layer, which forms thecore of the sandwich, is selected for its overall corrosion resistance,such as a steel alloy containing at least 12 percent chromium, commonlycalled stainless steel. Since. the structural requirements of the tankare met by the inner-v most or first layer, this core or b'arrierlayermay be relativelythin and is'preferably so in order to minimize the-cost:oflthe laminated material. The third or-outer-layer is selected to beanodic to the core or barrier layer, and is thus sacrificial to thebarrier layer. Again, since the structural requirements of the tank aremet by the first orinner layer, the thickness of the. outer layer may beselected strictly from the viewpoint'of'it's sacrificial action. Sincecarbonsteel is anodic to. stainlessv steel, and has a relatively'lowcost, this third or outer. layer, like the first layer, may be a mildorplain carbon steel. This sacrificial outer layer prevents, or at leastdelays, pitting cor-' rosion of the barrier layer, greatly extending theservice life of the tank. I

. 2 BRIEF DESCRIPTION OF THE DRAWlNGS The: invention maynbemore readilyunderstood when considered in view of the following detailed descriptionof exemplaryerribodiments thereof, taken in connection with theaccompanying drawings, inwhich:

FIG; lis aperspective view of an electrical distribution transformer ofthe residential distributionetype, which maybe 1 constructedaccording-to the teachings of the invention;

- FIG. 2 is a perspective'view-of an electrical transformer of thenetwork-type, disposed inanunderground vault, which may tion;

be constructed according to the teachings of the inven- FlG.'3 is afragmenta ry', sectional view of the tank or casing .ofelectrical'apparatus constructed according to an embodiment of theinvention; 7

FIGQ 4 is afragmentary, sectional view of a tank or casing of electricalapparatus constructed according to another embodiment of the invention;and

FIG. 5 is a fragmentary, sectional view of the tank shown in FIG. 2,modified to include a protective outer coating of insulating material.

DESCRIPTION OF PREFERRED EMBODIMENTS sive environments, such as belowgradelevel, and in general is 1 applicable to any electrical apparatusof this type having an electrical element disposed in a tank, with theelement being adapted for connection to an externalelectrical potential.

Electrical transformers filled with mineral oil. ask'arel, water, SFnorthe like, may utilize theteachings of the invention, as wellrascapacitors and protective ,apparatus, such as circuit breakers, whichrequire a corrosion resistant tank or casing.

FIGS-land Za're perspective 'views of electrical transformers l0 and 12,respectively, of the type which may be cohstrueted accordingtoztheteachings of the invention; Transformer 10 shown inFlG. .1 is adistribution transformer of the type commonly used forundergroundresidential distribution,

and it maybe disposed'in a vault 14 below grade level 16, asillustrated, it may be disposed in a vault which'is partially belowgrade level, or it may be directly buriedin the earth without thebenefit of a surrounding'vault, as dictated by-the require- .mentsof theelectrical utility. Transformer 10 includes a casing 18. having a cover20 which encloses the core-winding as sembly (not shown) of thetransformer 10. Asuitable insulatinggan'd cooling fluid, such as mineraloil, is also disposed in casing.v 18, to aid in insulating and coolingthe electrical:

windings of the transformer. Transformer 10 is hermetically; sealed;with the electrical connections 'to theencased high voltage windingbeing made through the sealed high voltage.

bushing-connector assemblies 28 and 30, and to thelowvoltagewindingthrough the sealed low voltage bushing assembly 1 32.The vault 14 has a heavy access'cover 34, which may be, 55

perforated to aid in the removal of heat from the transformer 10.-

Transformer 12 shown in FIG. 2 is a power transformer of the networktype, installed in a vault 40 which has a pluralityv of access coversthereon, such as cover 42. The top of vault40 is'-usually at grade level44. Transformer 12 includes the main transformersection 46, a highvoltage switch 48, and a .network protector 50. An external high voltageelectrical potential. is connected to the high voltage switch 48 througha multiple conductor single bushing 52, or through separate bushings,and the lower voltage distribution potential is obtained fromthesecondary bushings, such as secondary bushing v54. The maintransformer portion 46 includes a tank 56 which encloses thecore-windingassembly of the transformer, and also an insulating and cooling fluid,such as mineral oil or askarel.

Transformers l0 and l2are both subjected to highly corrosiveenvironments, and must withstand flooding of their respective vaultswith corrosive, polluted water for extended periods of time. Suchenvironments have caused the failure of electrical transformers due totank corrosion in periods much shorter than their normally expectedservice life.

Excellent organic coatings for underground transformers have beendeveloped, but since coatings are not always pinhole free, and sincecoatings may be scratched during the handling and installation of theelectrical apparatus, coatings alone do not provide the desired answer.Transformers and other fluid filled electrical apparatus only requireone hole through the casing or tank to cause costly damage and evenfailure of the apparatus. The fact that 99 percent of the casing may becorrosion free is of no benefit if one small portion of the casing isseverely attached by corrosion. in fact, an excellent protective coatingwith one scratch may cause failure of the tank at the exposed areafaster than if the tank had no protective coating.

Constructing the tanks of corrosion resistant materials, such asstainless steel, is not desirable because of the economic penalty, andalso because stainless steel, while not generally susceptible tocorrosion may, in certain environments, be subject to a localizedpitting corrosion which is very rapid, perforating the casing at thepoint of attack while the majority of the surface is corrosion free.This localized attack may also promote stress-corrosion cracking ofstainless steel which may cause failure of the tank even beforeperforation due to pitting occurs.

Broadly, the present invention is new and improved fluid filledelectrical apparatus having a tank formed of a laminated structure. Thelaminated tank structure includes at least first, second and thirdmetallic layers, which will also be referred to as the inner, core andouter layers, respectively, from the viewpoint of the tank or casingconstruction. In other words, the external surface of the inner layer isadjacent to the fluid inside the casing, the external surface of theouter layer is adjacent to the environment in which the electricalapparatus is placed, and the core layer is sandwiched between the innerand outer layers.

Laminated metals for reducing corrosion are known in the prior art, butthey have either been unsuccessful in arresting certain types ofcorrosion, too costly, or both. For example, cladding the externalsurface of a core material is known in which a relatively thick mildsteel core has thin layers of stainless steel disposed on its two majoropposed surfaces. This approach, however, at least for electricalapparatus disposed underground, suffers from the functional disadvantageof tanks formed of stainless steel, as the thin layers of stainlesssteel are subject to pitting and stress corrosion, exposing the mildsteel core. Another example of cladding known in the art utilizes arelatively thick core of stainless steel, clad with thin layers of steelcontainingabout 12 percent chromium. While this structure may besuitable functionally for the tanks of underground electrical apparatus,it is not economically attractive.

FIG. 3 is a fragmentary, cross-sectional view of fluid filled electricalapparatus having a tank or casing 60 constructed according to theteachings of the invention. Casing 60 may be the tank 18 or cover 20 oftransformer shown in FIG. 1, the tank 56 of transformer 12 shown in FIG.2, or the casing of any fluid filled electrical apparatus having anelectrically conductive member therein adapted for connection to anelectrical potential. Casing 60 has a first or inner layer 62 adjacentto the fluid 64, a second or core layer 66 and a third or outer layer68. The present invention economically solves the corrosion problem ofunderground electrical apparatus by selecting the material and thicknessdimension of the first layer 62 only for its mechanical properties andcost, without regard to its ability to resist corrosion. The thicknessof layer 62 is selected to provide at least the structural requirementsof the casing while it is in service. For distribution transformers ofthe residential type, this layer may be about 0.030 to 0.050 inch thick,while power transformers of the network type may have a first or innerlayer of about 0.100 to 0.250 inch thick. Since.

the mild or carbon steel commonly used for tanks of electrical apparatusmounted above grade, such as S.A.E. l0l0,is economically attractive, andpossesses the requisite mechanical properties for forming the tanks, thefirst layer is preferably a mild or plain carbon steel.

The second or core layer 66 of casing 60 is selected for its ability toresist corrosion and protect the structural layer 62. One of thestainless steels'i.e., noncorroding alloys of iron and chromium,including at least 12 percent chromium in order to produce the requiredpassivity, may be used. For example, suitable stainless steels are theA181 types 304 and 308, or the 18-8 stainless steels, such as AlSI type302. Since stainless steel is relatively costly, compared with carbonsteel, the thickness of the second layer 66 should only be that requiredto protect the structural layer. Thicknesses in the range of about 0.001to 0.020 inch are suitable, .with the thickness selected depending uponthe thickness of the structural layer 62. For example when thestructural layer is about 0.030 to 0.050 inch thick, the core layer 66may be about 0.006 inch thick.

Stainless steel, while not subject to general attack by the usualenvironments surrounding underground electrical apparatus, is subject tolocalized attack, called pitting corrosion, which may be very severe,causing rapid penetration, especially when subjected to environmentscontaining chlorides.

. Chlorides promote the formation of active-passive electrolytic cellsbetween the large passive or cathodic area and the small anodic areabeing attacked. Further, severe localized attack may cause failure ofthe core layer before perforation, as it promotes cracking of thestainless steel due to stress corrosion.

The function of the third or outer layer 68 is to protect the core layer66 from pitting and stress corrosion. lt accomplishes these functions byforming a sacrificial anode for the core layer, and as such the materialof which the outer layer is formed must be higher in the galvanic seriesthan the material of which the core layer is formed. Since stainlesssteel is the preferable material for the core layer 66, the outer layermay be formed of such materials as carbon steel, magnesium, aluminum, orzinc. In order to prevent the outer layer from being sacrificed toorapidly, however, it will usually be preferable to select a materialwhich'is higher than the core material in the galvanic series, but nottoo much higher. Thus, carbon steel, which is above, but close tostainless steel in the galvanic series, is an excellent material for thethird layer 68, and it additionally has the benefit of being relativelylow in cost.

The thickness dimension of the third layer 68 depends upon thecorrosiveness of the intended environment. Since the I outer layer willbe sacrificed, it'eventually will provide reduced structural strengthfor the casing and its thickness need not be influenced greatly bystrength requirements. As hereinbefore stated, the inner layer 62 isselected to provide adequate structural requirements of the casing whilethe transformer is in service. The third layer 68 will provideadditional structural strength for the casing during manufacturing andshipment of the transformer, when it requires the greatest strength.Thus, the inner layer 62 must be selected to at least provide thein-service strength requirements of the casing, and the inner and outerlayers together must provide adequate structural strength formanufacture and shipment.

When theouter layer 68 is attacked to the point where the core orbarrier layer is exposed, the outer layer 68 will continue to beattacked in preference to the core layer 66 due to their relativelocations in the galvanic series, thus stopping the penetration of thecorrosion through the tank wall. The outer layer will continue to besacrificial in preference to the core layer, as long as the area of thethird layer material exposed by the corrosion, i.e., the sides of thecraters produced by the corrosion, plus a predetermined surfacearea ofthe third layer immediately adjacent the crater, such as about a 2 inchradius about the center of the corrosive attack for carbon steel, isgreater than the area of the core material exposed by the corrosion.This is why the thickness of the outer layer is selected with thecorrosiveness of the intended environment in mind. If the third layer isselected to be a metal higher in the galvanic series than carbon steel,such as zinc, more core area may be exposed before the core layer isattacked.

In the example shown in FIG. 3, the first and third layers 62 and 68 areshown to have the same thickness dimension, but FIG. 4 illustrates afragmentary sectional view of a tank or casing in which the firstandthird layers have different dimensions. Like reference numerals inFIGS. 3 and 4 indicate like components, and like reference numerals witha prime mark indicates similar but modified components.

More Specifically, FIG. 4 illustrates a tank 60' having first and secondlayers 62 and 66, such as illustrated in FIG. 3, but having a thirdlayer 68 which is substantially thinner than the third layer 68 shown inFIG. 3, as functionally itwill usually not be necessary to make thethird layer as thick as the structural layer. inch FIG. 5 is afragmentary, cross-sectional view of the tank60 shown in FIG. 3,including a protective coating 70 disposed on the outer surface of thethird layer 68. Protective coating 70 may be selected from a largenumber of organic coatings specially developed to protect metals againstcorrosion, such as coatings formed of the epoxies, acrylics, asphaltics,alkyds, or polyesters. The thickness of the outer protective coatingdepends upon the particular coating selected, with about 0.001 to 0.020inch being the usual range. With the laminated tank constructiondisclosed herein, however, it would be suitable to use one of the lowercost protective coatings, such as an alkyd, which allows a slowdiffusion of moisture. A coating which alwhich apparatus would beseverely damaged or rendered uselows a slow diffusion of moisture isactually better than one i which is more moisture resistant, as pinholesand scratches in g the coating will not cause the'underlying metal to beas severely and rapidly attacked as would occur at openings in a moreperfect coating.

FIG. 5 also illustrates a corrosion crater 72 in the tank 60, and showshow the crater is stopped by the barrier layer 66. The corrosionpenetrates the outer layer 68 through an open ing in the coating 70, andproceeds inwardly until the barrier layer 66 is exposed. At this point,the outer layer 68 becomes sacrificial to the barrier layer 66, and thecorrosion attacks the outer layer, usually along the surface of thebarrier layer, enlarging the size of the crater without pitting orotherwise attacking the barrier layer. As long as the area of the wall74, plus a predetermined surface area about the crater, does not exceedthe area 76 of the barrier layer which is exposed, the outer layer willcorrode in preference to the barrier layer.

The laminated structure of which the casings or tanks are formed may beproduced by any suitable method, such as by heating metal slabs of thelayer materials to welding temperature, and then rolling them togetherto bond the slabs together.

The laminated material, when being fabricated into the tank or casingstructure, may be welded by conventional methods, with stainless steelwelding rods generally being preferable if the core layer is stainlesssteel, although mild steel rods may also be successfully used. Ingeneral, it is preferable that the third or sacrificial layer be atleast as sacrificial to the welding material used as it is to the corelayer. Welded joints in which the core layers are tied together via theweld material are most desirable, but overlap welds are suitable if theoverlap dimen- I ing in highly corrosive environments, such asunderground,

less in the event of a perforation in its tank or casing. The electricalapparatus includes a tank or casing constructed of a laminated metallicstructure which greatly extends the useful service life of the tank,without undue economic penalty. The tank material includes three layersof metal, with the outer layers functioning properly even if constructedof plain carbon steel, such as commonly used for tanks and casings ofsimilar apparatus mounted in noncorrosive environments, and the corematerial, which may be selected from one of the stainless steels, whilebeing more costly per pound than carbon steel, is a relatively thinlayer, such as form about 0.001 to 0.020 inch.

The additional cost of the thin layer of core material, and the cost ofproducing the laminated structure, IS substantially less than a tankconstructed completely of stainless steel, and is easier to fabricate.The additional cost of the laminated tank, compared with a tankconstructed of mild steel, is more than offset by the increased servicelife of the apparatus when disposed in corrosive environments.

I claim as my invention: 1. Electrical apparatus comprising: acasing, Ii an electrically conductive element disposed in said casing, saidelectrically conductive element being adapted for connection to anelectrical potential; and fluid means disposed in said casing, saidcasing being at least partially constructed from a laminated metallicstructure having at least first, second and third layers, said firstlayer being a structural layer, in

contact with said fluid means, said second layer being a barrier layer,disposed between. said first and third layers, to protect said firstlayer from corrosion, and said third layer being anodic to said secondlayer, forming a sacrificial anode to reduce pitting corrosion of thesecond layer.

2. The electrical apparatus of claim 1 wherein at least the first layeris a carbon steel and the second layer is a steel alloy containing atleast 12 percent chromium.

3. The electrical apparatus of claim 1 wherein at least the third layeris a carbon steel and the second layer is a steel alloy containing atleast 12 percent chromium.

4. The electrical apparatus of claim 1 wherein the first and thirdlayers are carbon steels and the second layer is a stainless steel.;

5. The electrical apparatus of claim 1 wherein the first layer issubstantially thicker than the second layer. r

6. The electricalapparatus of claim 1 wherein the secon layer has athickness dimension in the range of about 0.001 to 0.020 inch.

7. The electrical apparatus of claim 1 including an insulating coatingdisposed on the third layer.

8. The electrical apparatus of claim 1 wherein the first and thirdlayers are carbon steels and the second layer is a stainless steel, andthe first layer is substantially thicker than the second layer.

9. The electrical apparatus of claim 1 wherein the first and thirdlayers are carbon steels and the second layer is a stainless steel, andthe second and third layers are each thinner than the first layer. 7

1. Electrical apparatus comprising: a casing, an electrically conductiveelement disposed in said casing, said electrically conductive elementbeing adapted for connection to an electrical potential; and fluid meansdisposed in said casing, said casing being at least partiallyconstructed from a laminated metallic structure having at least first,second and third layers, said first layer being a structural layer, incontact with said fluid means, said second layer being a barrier layer,disposed between said first and third layers, to protect said firstlayer from corrosion, and said third layer being anodic to said secondlayer, forming a sacrificial anode to reduce pitting corrosion of thesecond layer.
 2. The electrical apparatus of claim 1 wherein at leastthe first layer is a carbon steel and the second layer is a steel alloycontaining at least 12 percent chromium.
 3. The electrical apparatus ofclaim 1 wherein at least the third layer is a carbon steel and thesecond layer is a steel alloy containing at least 12 percent chromium.4. The electrical apparatus of claim 1 wherein the first and thirdlayers are carbon steels and the second layer is a stainless steel. 5.The electRical apparatus of claim 1 wherein the first layer issubstantially thicker than the second layer.
 6. The electrical apparatusof claim 1 wherein the second layer has a thickness dimension in therange of about 0.001 to 0.020 inch.
 7. The electrical apparatus of claim1 including an insulating coating disposed on the third layer.
 8. Theelectrical apparatus of claim 1 wherein the first and third layers arecarbon steels and the second layer is a stainless steel, and the firstlayer is substantially thicker than the second layer.
 9. The electricalapparatus of claim 1 wherein the first and third layers are carbonsteels and the second layer is a stainless steel, and the second andthird layers are each thinner than the first layer.